During the course of studies of the metabolism of phenylalanine, measurements of oxygen uptake by liver preparations after addition of phenylalanine were used to estimate the capacity for phenylalanine hydroxylation in vitro. The potential for using this system as a method for routine measurement of the rate of phenylalanine hydroxylation will be investigated. The kinetic characteristics of the system will be examined in detail by determining changes in the rate of oxygen consumption and tyrosine formation over time with a range of concentrations of phenylalanine and pteridine cofactor in the incubation medium. Information about the kinetic characteristics of the system should indicate whether it will provide a reliable and reproducible rate assay for the enzyme. Investigation of nutritional and hormonal factors that influence the catabolism of branched-chain amino acids by muscle will be investigated using the isolated perfused rat hindquarter. Rates of conversion of leucine, isoleucine and valine to their keto analogs and to CO2 will be measured using hindquarters from diabetic rats perfused with media containing different quantities of the amino acids, with and without insulin. Changes in concentrations of amino acids in the medium will also be measured. The capacity of muscle for transamination of other amino acids and the extent to which amino acids compete with each other for transamination will also be examined.